KVM: arm64: Make kvm_call_hyp() a function call at Hyp

kvm_call_hyp() has some logic to issue a function call or a hypercall
depending on the EL at which the kernel is running. However, all the
code compiled under __KVM_NVHE_HYPERVISOR__ is guaranteed to only run
at EL2 which allows us to simplify.

Add ifdefery to kvm_host.h to simplify kvm_call_hyp() in .hyp.text.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210319100146.1149909-9-qperret@google.com